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6.3 Advanced Current Mirrors: Wide-Swing Q3 and Q4 Acts Like a Single-Diode Connected Transistor to Create the Gate Source Voltage for Q3

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6.3 Advanced Current Mirrors: Wide-Swing Q3 and Q4 Acts Like a Single-Diode Connected Transistor to Create the Gate Source Voltage for Q3 6.3 Advanced current mirrors: wide-swing Q3 and Q4 acts like a single-diode connected transistor to create the gate source voltage for Q3. Including Q4 helps lower the Vds3 so that it matches Vds2. Other than that, Q4 has little effect on the circuit’s operation. Assume ID2=ID3=ID5 VGS1=VGS4 If n=1 Also we need Chapter 6 Figure 12 6.3 Advanced current mirrors: wide-swing In most applications, it is desirable to make (W/L)5 smaller than that given in the Figure so that Q2 and Q3 can be biased with a slightly larger Vds. This would help counter the body effect of Q1 an Q4, which have their Vt increased. To save power consumption, Ibias and Q5 size can be scaled down a little bit while keeping the same gate voltage. Also, it may be wise to make the length of Q3 and Q2 larger than the minimum and that of Q1 and Q4 even larger since Q1 often sees a larger voltage Vout. This helps reduce short- channel effects. Chapter 6 Figure 12 6.3.2 Enhanced output impedance CM and Gain boosting The basic idea is to use a feedback amplifier to keep the drain-source voltage across Q2 as stable as possible, irrespective of the output voltage. From small-signal analysis, Ix=gmvgs+(Vx-Vs)/rds1, Vgs+Vs=A(0-Vs), Vx=Ix*rds2 Note that the stability of the feedback loop comprised of A and Q1 must be verified. Chapter 6 Figure 13 6.3.2 Enhanced output impedance CM and Gain boosting This technique can also be applied to increase the Rout of a cascode gain stage (the small signal current –gm2vin must go through Rout and CL). Comparing the DC gain only, it can be seen that it is a factor of (1+A) larger than the conventional cascode amplifier discussed in Chapter 3. To realize this gain, note that the Ibias current source must be similarly enhanced to achieve comparable output impedance as Rout. Chapter 3 Figure 16 Chapter 6 Figure 14 6.3.2 Sackinger’s design The feedback amplifier in this case is realized by transistor Q3 and Q1. Note that Q3 is a CS amplifier, therefore the gain is gm3rds3/2 if IB1 has an output impedance of rds3. So the total output impedance from the drain of Q1 is: The circuit consisting of Q4, Q5 and Q6, Iin and IB2 operates likes a diode-connected transistor, but its main purpose is to match those transistors in the output circuitry so that all transistors are biased accurately and Iout=Iin. One major limitation is that the signal swing is significantly reduced due to Q2 ad Q5 being biased to have drain-source voltages much larger ( ) Chapter 6 Figure 15 6.3.3. Wide-swing current mirror with enhanced output impedance Such a circuit is very similar to the Sachinger’s design, except that diode-connected transistors used as level shifters Q4 have been added in front of the CS amplifiers. The current density of most transistors (except Q3 and Q7) are about the same, Veff, and that of Q3, Q7, 2Veff. So Two issues with this circuit: 1. power consumption may be large, 2 additional poles introduced by the enhanced circuitry may be at lower frequencies. Chapter 6 Figure 16 6.3.3. Wide-swing current mirror with enhanced output impedance A variation of the previous circuit is shown below. It reduces the power, but matching is poorer. Note that Q2 in previous circuit is split to Q2 and Q5 in this circuit. It is predicted that this current mirror may be more used when power supply voltage is smaller or larger gains are desired. Chapter 6 Figure 17 6.3.4 Summary of improved current mirrors When using the OpAmp-enhanced current mirrors, it may be necessary to add local compensation capacitors to the enhancement loops to prevent ringing during transients. Also, the settling time may be increased (to tradeoff with large gain). Many other current mirrors exist, each having its own advantages and disadvantages. Which one to use depends on the requirements of the specific application. OpAmps may be designed using any of the current mirrors, therefore we can use the following symbol without showing the specific implementation of the current mirror. Just one specific implementation of the current mirror in (a) Chapter 6 Figure 18 6.4 Folded-cascode OpAmp Many modern OpAmps are designed to drive only capacitive loads. In this case, it is not necessary to use a voltage buffer to obtain a low output impedance. So it is possible to realize OpAmps with higher speeds and larger signal swings than those that drive resistive loads. These OpAmps are possible by having only a single high-impedance node at the output. The admittance seen at all other nodes in these OpAmps are on the order of 1/gm, and in this way the speed of OpAmp is maximized. With these OpAmps, compensation is usually achieved by the load capacitance CL. As CL gets larger, these OpAmps gets more stable but also slower. One of the most important parameters of these modern OpAmps is gm (ratio of output current over input voltage), therefore they are sometimes referred to as Operational Transconductance Amplifiers (OTA). A simple first order small-signal model for an OTA may be shown below: Chapter 6 Figure 19 Folded-cascode OpAmp A differential-input single-ended output folded-cascode OpAmp is shown below. The current mirror in the output side is a wide-swing cascode one, which increases the gain. The basic idea of the FC-OpAmp is to apply cascode transistors to the input differential pair but using transistors opposite in type from those used in the input stage. (i.e. Q1, Q2 nMOS and Q5, Q6 pMOS). This arrangement allows the output to be the same as the input bias voltage. The gain could be large due to large output impedance. If even larger gain is desired, one can use gain-enhancement techniques to Q5-Q8 as described in 6.3.2. Chapter 6 Figure 20 Folded-cascode OpAmp The single-ended output FC-OpAmp can be converted to a fully-differential one (to be detailed later). A biasing circuit can be included to replace Ibias1, Ibias2 and connect to VB1 and VB2. The two extra transistors Q12 and Q13 can increase slew rate performance and prevent the drain voltages of Q1 and Q2 from having large transients thus allowing the OpAmp to recover faster following a slew rate condition. Chapter 6 Figure 20 The compensation is realized by the load capacitor CL (dominant pole compensation). DC biasing: note ID3/4=ID1/2+ID5/6 When CL is small, it may be necessary to add additional capacitors in parallel with the load. If lead compensation is to be used, then a resistor is in series with CL. 6.4.1 Small-signal analysis In small-signal analysis, the small-signal current from Q1 goes directly from source to drain and to CL, while that of Q2 indirectly through Q5 and current mirror of Q7-Q10 to CL. (assuming 1/gm5/6 much larger than rds3 and rds4). Note that these small-signal currents go through different path to the output, therefore their transfer function are different (due to the pole/zero caused by the current mirror for small-signal current of Q2). However, usually, these pole/zero are much larger than the unity-gain frequency of OpAmp and may be ignored. So an approximate gain transfer function is: ZL is the parallel of impedance at drain of Q6, Q8, and CL. At high frequencies, Av is approximated as Chapter 6 Figure 20 6.4.1 Small-signal analysis The first-order model shows close to 90 degrees of phase margin. To maximize bandwidth, it is desirable to increase gm by using nMOS transistors, which means larger DC current on Q1/2 (Having large gm for Q1/2 also help reduce noise). Smaller currents on Q5/6 helps increase rout, which increases the DC gain. (the current ratio between them has a practical limit of 4 to 5.) For more detailed analysis, the second pole is associated with the time constants at the source terminals of Q5/Q6. At high frequencies, the impedance is on the order of 1/gm5/6, which in this case is relatively large due to smaller current. (so one can have larger currents in order to push this pole away and minimizing the capacitance is important too). Chapter 6 Figure 20 C. Lead compensation 6.4.2 Slew rate Diode-connected transistors Q12/13 are turned off during normal operation (as Vgd3/4=Vgs12<|Vtp|) and have almost no effect on the OpAmp. However, they improve the operation during slew rate limiting. If they are not present, then when slew rate occurs, all bias current of Q4 go to Q5 and out of CL through the mirror (at the same time Q6 conducts zero current in most cases). At this time, since all Ibias2 is diverted through Q1 and it is usually larger than ID3, both Q1 and Ibias2 go into triode region, causing Ibias2 to decrease until it is equal to ID3. As a result, the drain voltage of Q1 approaches ground. When OpAmp is back to normal operation, drain voltage of Q1 must slew back to the original biasing voltage, and this additional slewing increases distortion and transient delay. If Q12/13 were included, then when slew rate occurs (as the above case), Q12 conducts extra current from Q11 and also the current on Q3/4 increases, which eventually makes the sum of ID12 and ID3 equal to Ibias2.
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